Again, Quentin was already walking towards the mound. The situation had already escalated and from every video I have seen Greinke's initial response seemed to be a raise of his head and say something, not shout it. Quentin then started running towards the mound, by that point there was nothing anyone could do to stop the fight. Greinke did throw his glove aside.

There is a sort of macho element that was at play here obviously. I get the sort of psychological need for a pitcher to feel bulletproof and that he owns the plate or the mound or whatever. When a batter is charging the pitcher, he can't just run away. He shouldn't throw his glove aside and get ready to throw down though either, really all he can do is hope that the catcher can get in between him and the batter.